Stresses in rail track structures arise from various causes including changes of temperature, wheel/rail loads, changes in wheel/rail friction conditions during use. Contact stresses between wheel and rail are influential in determining rates of wheel and rail wear, track structure degradation and the initiation and growth of rolling contact fatigue cracks.
There are a number of key factors that play significant roles in determining the magnitudes of stresses occurring between wheel and rail, particularly when rail vehicles are negotiating curved track. These include rail vehicle weights and steering characteristics, train lengths, track gradient and curvature, as well as the state of friction between wheel and rail surfaces and the relationships between train speed, track geometry, and the distribution of locomotive power along the length of the train.
Included in the parameters associated with track geometry is so-called superelevation (or cant). Superelevation is introduced in curves to counteract the centrifugal force that is generated when a rail vehicle negotiates a curve of a given radius at a given speed. The theoretical speed at which this centrifugal force is perfectly balanced for a single wheelset negotiating a curve of specific radius and superelevation is referred to as the balance speed for the curve. Balance speed is an important concept in railway operations due to the fact that vehicles travelling significantly above balance speed in a curve face an increased probability of derailment while vehicles travelling significantly below balance speed can impart dramatically increased and (in the case of heavy axle load vehicles) destructive forces on the track structure.
Track design and (in many cases) ongoing maintenance involves specifying and establishing a specific superelevation for each curve in a track segment. The permissible operating speed in a given segment can be referred to as the posted speed. Rather than setting superelevation in each curve such that the balance speed is equal to the posted speed, the typical approach is to specify a lesser degree of superelevation so that the balance speed is a prescribed amount lower than the posted speed. While this is believed to improve vehicle steering at posted speed, the primary purpose is to accommodate a realistic distribution of speeds over a given track segment that will actually be lower than the posted speed.
Optimizing any strategy for specifying, establishing and maintaining superelevation in a curve therefore depends explicitly on accurate knowledge of the distribution of vehicle speeds for given train types operating in the curve. This information is often largely unknown or uncertain, and can vary significantly over time with changing traffic types and conditions. This can result in accelerated and unnecessary wear and track structure degradation when a curve is maintained with a degree of superelevation that is not matched to the realistic distribution of vehicle speeds.